On 12/18/2014 02:08 PM, Chris St. Pierre wrote:
I wasn't suggesting that we *actually* use filesystem link count, and
make hard links or whatever for every time the image is used. That's
prima facie absurd, for a lot more reasons that you point out. I was
suggesting a new database field that tracks the number of times an image
is in use, by *analogy* with filesystem link counts. (If I wanted to be
unnecessarily abrasive I might say, "This is a textbook example of
something called an analogy," but I'm not interested in being
unnecessarily abrasive.)
Overloading the protected flag is *still* a terrible hack. Even if we
tracked the initial state of "protected" and restored that state when an
image went out of use, that would negate the ability to make an image
I guess I don't understand what you consider to be overloading of the
protected flag. The original purpose of the protected flag was to
protect images from being deleted.
Best,
-jay
protected while it was in use and expect that change to remain in place.
So that just violates the principle of least surprise. Of course, we
could have glance modify the "original_protected_state" flag when that
flag is non-null and the user changes the actual "protected" flag, but
this is just layering hacks upon hacks. By actually tracking the number
of times an image is in use, we can preserve the ability to protect
images *and* avoid deleting images in use.
On Thu, Dec 18, 2014 at 5:27 AM, Kuvaja, Erno <[email protected]
<mailto:[email protected]>> wrote:
I think that’s horrible idea. How do we do that store independent
with the linking dependencies?____
__ __
We should not depend universal use case like this on limited subset
of backends, specially non-OpenStack ones. Glance (nor Nova) should
never depend having direct access to the actual medium where the
images are stored. I think this is school book example for something
called database. Well arguable if this should be tracked at Glance
or Nova, but definitely not a dirty hack expecting specific backend
characteristics.____
__ __
As mentioned before the protected image property is to ensure that
the image does not get deleted, that is also easy to track when the
images are queried. Perhaps the record needs to track the original
state of protected flag, image id and use count. 3 column table and
couple of API calls. Lets not at least make it any more complicated
than it needs to be if such functionality is desired.____
__ __
__-__Erno____
__ __
*From:*Nikhil Komawar [mailto:[email protected]
<mailto:[email protected]>]
*Sent:* 17 December 2014 20:34
*To:* OpenStack Development Mailing List (not for usage questions)
*Subject:* Re: [openstack-dev] [glance] Option to skip deleting
images in use?____
__ __
Guess that's a implementation detail. Depends on the way you go
about using what's available now, I suppose.____
__ __
Thanks,
-Nikhil____
------------------------------------------------------------------------
*From:*Chris St. Pierre [[email protected]
<mailto:[email protected]>]
*Sent:* Wednesday, December 17, 2014 2:07 PM
*To:* OpenStack Development Mailing List (not for usage questions)
*Subject:* Re: [openstack-dev] [glance] Option to skip deleting
images in use?____
I was assuming atomic increment/decrement operations, in which case
I'm not sure I see the race conditions. Or is atomism assuming too
much?____
__ __
On Wed, Dec 17, 2014 at 11:59 AM, Nikhil Komawar
<[email protected] <mailto:[email protected]>>
wrote:____
That looks like a decent alternative if it works. However, it
would be too racy unless we we implement a test-and-set for such
properties or there is a different job which queues up these
requests and perform sequentially for each tenant.____
__ __
Thanks,
-Nikhil____
------------------------------------------------------------------------
*From:*Chris St. Pierre [[email protected]
<mailto:[email protected]>]
*Sent:* Wednesday, December 17, 2014 10:23 AM
*To:* OpenStack Development Mailing List (not for usage questions)
*Subject:* Re: [openstack-dev] [glance] Option to skip deleting
images in use?____
That's unfortunately too simple. You run into one of two cases: ____
__ __
1. If the job automatically removes the protected attribute when
an image is no longer in use, then you lose the ability to use
"protected" on images that are not in use. I.e., there's no way
to say, "nothing is currently using this image, but please keep
it around." (This seems particularly useful for snapshots, for
instance.)____
__ __
2. If the job does not automatically remove the protected
attribute, then an image would be protected if it had ever been
in use; to delete an image, you'd have to manually un-protect
it, which is a workflow that quite explicitly defeats the whole
purpose of flagging images as protected when they're in use.____
__ __
It seems like flagging an image as *not* in use is actually a
fairly difficult problem, since it requires consensus among all
components that might be using images.____
__ __
The only solution that readily occurs to me would be to add
something like a filesystem link count to images in Glance. Then
when Nova spawns an instance, it increments the usage count;
when the instance is destroyed, the usage count is decremented.
And similarly with other components that use images. An image
could only be deleted when its usage count was zero.____
__ __
There are ample opportunities to get out of sync there, but it's
at least a sketch of something that might work, and isn't *too*
horribly hackish. Thoughts?____
__ __
On Tue, Dec 16, 2014 at 6:11 PM, Vishvananda Ishaya
<[email protected] <mailto:[email protected]>> wrote:____
A simple solution that wouldn’t require modification of
glance would be a cron job
that lists images and snapshots and marks them protected
while they are in use.
Vish____
On Dec 16, 2014, at 3:19 PM, Collins, Sean
<[email protected]
<mailto:[email protected]>> wrote:
> On Tue, Dec 16, 2014 at 05:12:31PM EST, Chris St. Pierre wrote:
>> No, I'm looking to prevent images that are in use from being deleted.
"In
>> use" and "protected" are disjoint sets.
>
> I have seen multiple cases of images (and snapshots) being
deleted while
> still in use in Nova, which leads to some very, shall we say,
> interesting bugs and support problems.
>
> I do think that we should try and determine a way forward on
this, they
> are indeed disjoint sets. Setting an image as protected is a
proactive
> measure, we should try and figure out a way to keep tenants from
> shooting themselves in the foot if possible.
>
> --
> Sean M. Collins
> _______________________________________________
> OpenStack-dev mailing list
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